This invention relates to a rapid method, a reagent and a device for detecting free available chlorine in aqueous fluids. More particularly, this invention concerns the use, as an indicator substance, of certain azine compounds which have unexpectedly been found to differentiate between free available chlorine and combined available chlorine in aqueous fluids.
The available chlorine family, which is of particular interest in the bleaching and disinfecting fields, is comprised of the chemicals which, when in aqueous solution, yield solutions of hypochlorous acid. These compounds are further subdivided into those which contain free available chlorine and those which contain combined available chlorine.
Free available chlorine describes chlorine in aqueous solution as hypochlorous acid, hypochlorite ion or, in strong acid solutions, as free chlorine. The use of free available chlorine as a disinfectant for swimming pool water and other water supplies has been popular for some time. Its popularity is due, in part, to its low cost, convenience and effectiveness as an antiseptic agent in relatively low concentrations. It is widely accepted that hypochlorous acid has the ability to diffuse through the cell wall of the bacterial cell and thereby cause the death of the cell. Since the amount of free available chlorine in an aqueous solution relates directly to the disinfecting or sanitizing activity thereof, a test which will rapidly and accurately measure free available chlorine has long been desired. This is particularly true since it is known that major disadvantages of free available chlorine are: (1) in higher than necessary concentrations, it is irritating to the eyes, (2) it has an unpleasant odor and (3) it rapidly dissipates from exposed water, either by escaping as a gas or by reacting with some component in water.
In the interest of adequate swimming pool sanitation and pleasant swimming, it is desirable to maintain the free available chlorine content of swimming pool water somewhere between the lowest effective concentration conducive to good sanitation and the highest concentration that is not irritating to the eyes. This range is fairly narrow, covering a concentration range of frome about 0.4 ppm. to about 2.0 ppm.
Combined available chlorine is mainly associated with organic chloramines which when in aqueous solution release only a small amount of free available chlorine. The balance of the chlorine is in a combined state which is not believed to be effective for disinfecting. Accordingly, systems which measure combined available chlorine together with free available chlorine yield results which are not truly indicative of the level of disinfectant in the solution tested.
In the past, oxidation-reduction indicators have been utilized for detecting chlorine in swimming pool and drinking water. However, it was known that such tests not only detected free available chlorine under ordinary conditions and use, but were also sensitive to combined available chlorine. Since the disinfecting activity of chlorine-containing compounds is due primarily to free available chlorine, it is extremely advantageous that a system for testing aqueous fluids for their bactericidal ingredients detect free available chlorine only, rather than free and combined available chlorine.
Exemplary of the prior art oxidation-reduction indicator systems which are still used to test aqueous fluids for chlorine are those which in the presence of chlorine change from one color to another, from a colored to a colorless material or from a colorless to a colored material. In the past materials such as o-tolidine, dimethyl-p-phenylenediamine (DPD), diphenylamine sulfate, naphthol, napthoflavone, o-toluidine, aniline and the like have been used in systems as chlorine indicators.
One oxidation-reduction indicator system which is used to measure free available chlorine and combined available chlorine is referred to as the orthotolidine-arsenite (OTA) method. In this method total chlorine is measured with orthotolidine (OT), a reading of color intensity being made five minutes after the addition of the OT to the water. Another test is made on a second sample of water by adding sodium arsenite thereto immediately following the addition of OT. It is critical that the sodium arsenite be added within 5 seconds of the first addition of OT, since the former reagent immediately reduces the combined available chlorine (chloramines) and prevents further reaction with OT. This latter test is read for color intensity and thereby gives a reading of free available chlorine in the sample. This test method and other test methods are more fully described in Sconce, J.S., "Chlorine, Its Manufacture, Properties and Uses," pp 475-477, New York, Reinhold Publishing Corporation, 1962. It can be seen from the foregoing that methods for the determination of free available chlorine have been complex and often required critical manipulative steps.